(a) Technical Field
The present invention relates to a mode change control method of a hybrid vehicle. More particularly, the present invention relates to a method of controlling the mode change from electric vehicle to hybrid electric vehicle.
(b) Background Art
A hybrid vehicle is a vehicle driven by at least two different types of power source. When the term ‘hybrid vehicle’ is used, it most often refers to a vehicle that is driven by an engine which generates a rotational force by burning a fuel (fossil fuel such as gasoline) and an electric motor which generates a rotational force with electric power of a battery to achieve better fuel efficiency with reduced exhaust gas.
FIG. 1 shows a schematic configuration of the hybrid vehicle. The hybrid vehicle includes an engine 10, an electric motor 20 (drive motor), and an automatic transmission 30. Particularly, the engine 10 and the drive motor 20 are connected to each other with an engine clutch 50 interposed therebetween, and the drive motor 20 and the automatic transmission 30 are connected to each other. An integrated starter-generator (ISG) for providing rotational force, i.e., cranking torque, to the engine 10 during start-up is connected to the engine 10.
The hybrid vehicle is driven by using only the power from the drive motor 20 during vehicle start-up or low speed driving operation. Since the engine efficiency is lower than the motor efficiency during initial start-up, it is advantageous that the initial start-up of the vehicle is performed using the drive motor 20 instead of the engine 10 in terms of fuel efficiency. After vehicle start-up, the ISG 40 starts the engine 10 so as to simultaneously use the engine power and the motor power.
In other words, the hybrid vehicle is driven in an electric vehicle (EV) mode, which is directed to a pure electric vehicle mode using only the rotational force of the drive motor 20, or in a hybrid electric vehicle (HEV) mode, which is an auxiliary mode using the rotational force of the drive motor as an auxiliary power source with the use of the rotational force of the engine as a main power source. A mode change from EV to HEV is achieved with the start-up of the engine by the ISG 40.
In order to perform torque control of the hybrid vehicle, it is necessary to accurately calculate a driver demand torque to meet the intention of the driver. For this purpose, an operation point determination circuit of a controller of the hybrid vehicle calculates an optimal operation point of the engine and the drive motor to perform a torque distribution control of the engine and the drive motor.
That is, since the driver demand torque is satisfied with “engine torque+motor torque,” the operation point determination circuit calculates torque command values of the engine and the drive motor based on the optimal operation points and outputs torque commands to the engine and the drive motor so as to satisfy the driver demand torque. Then, an engine controller and a motor controller control the engine and the drive motor to output torque corresponding to the torque command, respectively, thus satisfying the driver demand torque.
For example, when a high engine power is required (e.g., the driver fully depresses an accelerator pedal), the engine power is maintained at an optimal operation condition and any deficiency of power is supplemented by the motor. As a result, it is possible to drive the vehicle with high engine efficiency while satisfying the driver demand power.
On the other hand, when a low engine power is required (e.g., the driver slightly depresses the accelerator pedal), the engine power is reduced accordingly while the engine power other than the power required to drive the vehicle is used to generate electricity in the drive motor and the generated electricity is charged to a battery.
The mode change from EV to HEV in the hybrid vehicle is one of the most important functions and one of the factors that affect the driving performance, fuel efficiency and power performance of the hybrid vehicle.
Precise mode change control is essential and an optimal mode change algorithm in accordance with the driving state is necessary. In the mode change control, a clutch control is a major factor, and an accurate control of the clutch is related to the drive performance and the power performance of the hybrid vehicle.
A conventional method performs EV/HEV mode change in such a manner that a clutch pressure is feedback-controlled for a target slip amount [We (engine speed)−Wm (motor speed)]; however, it has the following problems.
In a case where the slip continues as transmission input speed (i.e. motor speed) is lower than engine idle speed such as when the vehicle is driven on a steep slope or a mild slope during idle creep, for the conventional method to control the clutch, it is necessary to: (a) accurately determine a gradient (additional hardware may be required); (b) determine an accurate vehicle load in consideration of the gradient; and, (c) define a target slip amount for all cases in consideration of the gradient and the vehicle load. In order to satisfy all of requirements (a), (b) and (c), controllers have to perform a considerable amount of operations, which entails various problems such as complex algorithm, excessive number of calibration variables, high CPU load, deterioration of driving performance and power performance, and unstable vehicle behavior.
Accordingly, there is a need for a method for performing the control by a more simplified process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.